In an internal combustion engine performing a fuel injection control by an electronic control unit (ECU), it is necessary to estimate an amount of air having flowed into each cylinder of the engine, in order to determine an amount of fuel to be injected from the injector. One of known ways to determine an air inflow into each cylinder is a D-jetronic method (a speed density method) which estimates the air inflow from a pressure (a negative pressure) within an intake pipe and a rotational speed of the engine.
In the situation that a throttle valve is abruptly operated, such as the case where a driver abruptly opens the throttle valve to accelerate the engine or the case where the driver abruptly closes the throttle valve to decelerate the engine, air-fuel ratio deviates from a suitable value because of a delay in response of an intake air drawn to the engine when a fuel of an arithmetically operated amount relative to the air inflow estimated from the intake pipe pressure and the rotational speed of the engine is supplied to the engine. If the engine is accelerated, the air-fuel ratio becomes too lean. On the other hand, if the engine is decelerated, the air fuel ratio becomes too rich. As a result, an exhaust gas composition may deteriorate, and so may deteriorate an operating performance of the engine, at the time of accelerating or decelerating the engine.
Therefore, when the throttle valve is abruptly performed in order to accelerate or decelerate the engine, the deterioration of the exhaust gas composition and the deterioration of the operating performance of the engine are prevented by correction to increase or decrease a fuel injection amount according to a changing amount of an opening degree of the throttle valve.
As a method for detecting an operation amount of the throttle valve, a throttle position sensor for detecting a position of the throttle valve is generally used; however, for reducing the cost, the throttle position sensor is not installed in many cases. In this case, it is required to detect the acceleration and deceleration of the engine without using the throttle position sensor.
As a method for detecting the acceleration and deceleration of the engine without using the throttle position sensor, there has been proposed a method for detecting an accelerating state or decelerating state of the engine when the intake pipe pressure is changed with a predetermined amount, as described in Japanese Patent Application Laid-Open No. 2002-242749.
In the method described in Japanese Patent Application Laid-Open No. 2002-242749, a plurality of crank angle positions of a crankshaft are predetermined in advance to be sampling positions for sampling intake pipe pressures, and each intake pipe pressure newly sampled at each sampling position is compared with a previous intake pipe pressure sampled at the same sampling position one combustion cycle before. Then, it is determined that the engine is in accelerating state when the newly sampled intake pipe pressure is higher by at least a predetermined level than the previously sampled intake pipe pressure and that the engine is in decelerating state when the newly sampled intake pipe pressure is lower by at least a predetermined level than the previously sampled intake pipe pressure.
A curve “Pb1” represented in a solid line in FIG. 6A shows variations of the intake pipe pressure in a steady state in which a four-stroke internal combustion engine is running at a substantially constant rotational speed. By contrast, a curve Pb2 represented in a broken line shows variations of the intake pipe pressure in an abruptly accelerating operation by opening the throttle valve at the position of a crank angle θ1. Thus, when the engine is being accelerated, the opening of the throttle valve results in a pressure rise in the intake pipe. Therefore, the accelerating state of the engine can be determined by comparing the intake pipe pressure sampled at each sampling position with the previous intake pipe pressure sampled at the same sampling position one combustion cycle before, and then, by detecting that the newly sampled intake pipe pressure is higher by at least a predetermined level than the previously sampled intake pipe pressure. The degree of acceleration can be determined by the rate of variation over time of the difference between the newly sampled intake pipe pressure and the intake pipe pressure sampled at the same position one combustion cycle before.
A curve “Pb3” represented in a broken line in FIG. 6A shows variations of the intake pipe pressure when the throttle valve is closed to perform abrupt deceleration. As seen from the drawing, when the engine is being decelerated, the closing of the throttle valve (a decrease of the air inflow to the intake pipe) results in a pressure drop in the intake pipe. Therefore, the decelerating state of the engine can be determined by comparing the intake pipe pressure sampled at each sampling position with the previous intake pipe pressure sampled at the same sampling position one combustion cycle before, and then, by detecting that the newly sampled intake pipe pressure is lower by at least a predetermined level than the previously sampled intake pipe pressure. The degree of deceleration can be determined by the rate of variation over time of the difference between the newly sampled intake pipe pressure and the intake pipe pressure sampled at the same position one combustion cycle before.
In the detecting method described in Japanese Patent Application Laid-Open No. 2002-242749, an AC voltage as shown in FIG. 6B, which is generated from a power generating coil inside of a magneto AC generator provided so as to rotate synchronously with the engine, is used as a rotational angle detection signal. Also, in this method, there is provided a signal generator which detects an edge of a reluctor provided on a rotor synchronously rotating with the engine and generates pulse signals Vp1 and Vp2 (FIG. 6C) at a specific crank angle position of the engine. The pulse signal Vp2 generated by the signal generator is specified as a reference crank angle signal, and a zero cross point of the rotational angle detection signal emerging immediately after the generation of the reference crank angle signal is specified as a sampling position “a”. Zero cross points, each emerging during following combustion cycles, are specified as sampling positions b, c, . . . , w, x, and the intake pipe pressure are sampled at these sampling positions (24 sampling positions in the drawing). In the shown example, a position where the reference crank angle signal Vp2 is generated is set at a position slightly advanced from a top dead center position at the time of starting a suction stroke. The top dead center position is a crank angle position when a piston of the engine reaches the top dead center.
According to the method for detecting acceleration/deceleration disclosed in Japanese Patent Application Laid-Open No. 2002-242749, it is possible to detect that the engine is in accelerating and in decelerating state without using the throttle position sensor. However, in the case where the determination of the accelerating/decelerating state is performed based on the sampled intake pipe pressure at each sampling position, the following problem occurs.
The intake pipe pressure abruptly decreases in the suction stroke, the minimum value is shown at the end of the suction stroke and the beginning of a compression stroke. After the minimum value of the intake pipe pressure is shown, the intake pipe pressure gradually increases until a point immediately before the next suction stroke. A degree of increase in an increasing process of the intake pipe pressure is controlled by the time constant decided by the opening degree of the throttle valve (an opening area) and volume inside of the intake pipe provided between the throttle valve and an intake valve (volume inside of the intake pipe on the downstream of the throttle valve). In the increasing process of the intake pipe pressure, since the intake valve is closed, the intake pipe pressure is not affected by a piston movement (the crank angle).
In addition, in the increasing process of the intake pipe pressure, the increasing speed of the intake pipe pressure becomes low when the opening degree of the throttle valve is small, since an amount of air flowing through the opening area of the throttle valve is little, and the increasing speed of the intake pipe pressure becomes high when the opening degree of the throttle valve is large, since the amount of air flowing through the opening area of the throttle valve is large. After the suction stroke is finished, the intake valve is closed in the increasing process of the intake pipe pressure; therefore, the intake pipe pressure is irrelevant to the crank angle. Because of the constant volume of the intake pipe, the intake pipe pressure after the suction stroke is determined by the opening area of the throttle valve and the elapsed time.
However, in the already proposed accelerating/decelerating state detection method, after the suction stroke is finished, the intake pipe pressure is sampled at the predetermined crank angle position which is used as the sampling position even in the increasing process of the intake pipe pressure. Accordingly, in a transient state where the rotational speed of the engine is changing, the intake pipe pressure sampled at each sampling position and the intake pipe pressure sampled at the same sampling position one combustion cycle before become unrelated as the object of comparison, which causes a problem that detecting accuracy for the accelerating/decelerating state becomes worsen.